Adverse drug interactions (ADI), are a major public health problem in the US. A major contributor to both ADI and failure of therapeutic efficacy is genetic variability and/or environmentally-induced (incl. diet) changes in hepatic and intestinal drug disposition. Cytochrome P450 3A4 (CYP3A4) is responsible for the hepatic and intestinal metabolism of numerous xenobiotics, including pharmaceutical compounds, some carcinogens, and several important endogenous steroids. Constitutive and inducible expression of CYP3A4 is regulated by a variety of different transcription factors/pathways. Among the most important determinants of CYP3A4 activity is the hormone nuclear receptor, Steroid and Xenobiotic Receptor (SXR;also known as the Pregnane X-receptor, PXR). Numerous drugs and non-drug chemicals induce CYP3A4 activity by acting as SXR ligands, and can cause significant adverse drug-drug interactions and alter therapeutic efficacy. In studies initially investigating the chemopreventive actions of the dietary phytochemical, sulforaphane (SFN;an isothiocyanate found in broccoli), we found that SFN causes remarkable 'down-regulation of CYP3A4 in human hepatocytes. We then verified this effect in human-derived intestinal cells (LS-180), and then demonstrated that SFN acts as an effective antagonist of ligand binding to SXR at low micromolar concentrations. Additional studies demonstrated that this is a species (human)-specific effect, consistent with known differences in SXR ligands between rodents and humans. In this grant we propose to: 1) further elucidate the molecular mechanisms by which SFN blocks SXR function;2) evaluate structural analogs of SFN to identify more potent and/or specific SXR antagonists;3) utilize 'humanized'SXR mice and other in vivo approaches to establish an animal model for human response, and 4) conduct a series of human feeding studies to determine if SFN in broccoli sprouts lowers basal expression of human CYP3A4 and/or interferes with rifampicin-mediated induction of CYP3A4, measured by midazolam clearance. If the basic feeding study identifies an effect on CYP3A4, additional feeding studies are proposed to examine other SFN analogs and further characterize the dose and time course of effects. These results may lead to the development of important new therapeutic and dietary approaches that could reduce adverse drug responses. They could also help explain the large inter-individual variability in CYP3A4 that has been noted for decades. Finally, they could help to further elucidate the risks and benefits of sulforaphane, a potential dietary phytochemical that has been proposed as a safe 'nutraceutical'compound to reduce cancer risk.